1. Field of the Invention
The present invention relates to a laser processing method using pulse laser, in particular, using a ultra-short pulse laser whose pulses have a temporal width of 10xe2x88x9212 second or less such as femto second (10xe2x88x9215 second) pulses, and to a processing apparatus for performing the processing method.
2. Description of the Related Art
Recently, there is rapidly growing interest in a processing technology making use of a pulse laser, in particular, ultra-short pulse laser having femto second pulses. However, when the ultra-short pulse laser having a wavelength band width is focused using an ordinary refraction single lens, chromatic aberration occurs due to the refractive dispersion of a refraction lens (a property that the refractive index depends on wavelength, thereby a focused spot is extended laterally and longitudinally. The chromatic aberration depends on the wavelength bandwidth of the ultra-short laser pulse and the magnitude of the dispersion of the refraction lens.
Further, the pulse width of the ultra-short pulse laser is extended by the dispersion of the refraction lens. The magnitude of the pulse extension depends on the peak intensity of pulses, in addition to that it depends on the wavelength bandwidth of the pulse and the dispersive characteristics of the lens material.
Accordingly, when the ultra-short pulse laser is applied to processing, it is necessary to make the processing practically usable by correcting chromatic aberration, or by correcting both chromatic aberration and pulse extension in some cases.
Further, as shown in, for example, FIG. 1, there is also known a processing method of branching the pulse beam from a laser generator 1 into a plurality of pulse beams using a diffraction element 2, focusing the plurality of branched pulse beams using a refraction type focusing lens 3 disposed behind the diffraction element 2, and impinging the focused pulse beams on a work 4. The diffraction element 2 is designed such that the phase thereof is distributed to provide the respective branched beams with substantially the same intensity.
Further, the focusing lens 3 has a Fourier transform action and focuses the plurality of branched pulse beams. In general, there is the following relationship between a pulse width xcex94t of a pulse and a wavelength width xcex94xcex of a pulse.
xcex94txc2x7xcex94xcexxe2x89xa7Cxe2x80x83xe2x80x83(1) 
where, C shows constant. It can be found from the expression (1) that a pulse having a shorter pulse width has a larger wavelength band width. Note that for femto second pulses, a 100 femto second pulse has a wavelength band width of up to xc2x110 nm.
When the wavelength band width of a pulse is shown by xcex94xcex, the positional derivation (chromatic aberration) xcex94h of laser beams branched by the diffraction element 2 can be estimated by the following relation (2).
xcex94h=(mf/p)xcex94xcexxe2x80x83xe2x80x83(2) 
where, m shows the order of diffraction, f shows the focal length of a lens, p shows the length of one cycle of the diffraction element 2. Since the pulse beams branched by the diffraction element 2 are extended as described above, the diameter of a hole to be processed at the position where the branched beam is focused is also extended in the diffraction direction. While the case in which the diffraction element is arranged one-dimensionally is described here, this is also applied to the case in which it is arranged two-dimensionally.
It can be understood from the relation (2) that when the femto second pulse is branched using the diffraction element, it is difficult to focus the pulses branched by the diffraction element into a small spot because a large chromatic aberration occurs due to the large wavelength band width of the femto second pulse. In contrast, in a long pulse, no problem with the above chromatic aberration occurs even if the pulse beam is branched by the diffraction element because the wavelength band width of the pulse is negligible.
When the femto second pulse laser is branched into the plurality of pulse beams using the optical system shown in FIG. 1 and the pulse beams are focused, the focused spots of the higher-order diffractived beams are extended in terms of shape in the diffraction direction as shown in FIG. 2 due to the wavelength band width of the pulse. Therefore, when the array of the focused spots is impinged on the work 4, processed marks due to higher-order beams are deformed in an oval shape different from the shape of processed marks from lower-order beams. Accordingly, it is difficult to perform the processing uniformly at a plurality of positions.
The femto second laser pulse used in the experiment of branching and focusing using the optical system shown in FIG. 1 has a pulse width of 100 fs, a pulse energy of 1 mJ (repetition rate of 1 KHz), a center wavelength of 800 nm, and a half wavelength bandwidth of xc2x110 nm.
The objective of the present invention, which was made in view of the above problems, is to provide a laser processing method and a laser processing apparatus capable of performing the processing more precisely by correcting chromatic aberration or correcting both chromatic aberration and pulse extension when the processing is performed with a single beam of the pulse laser or when the processing is performed with a plurality of beams obtained by branching the laser pulse laser.
In view of the above problems, the present invention employs the following methods and arrangements.
(1) In a laser processing method of the present invention for processing a work by focusing a laser pulse and impinging the focused laser pulse on the work, chromatic aberrations pertinent to the laser pulse are corrected by the combination of diffractive and refractive optical elements. According to the method, diffractive-dispersion and refractive-dispersion are caused by the diffractive and refractive optical elements, respectively. These two kinds of dispersion materializes in a counter-opposite manner and the magnitude of the diffractive-dispevsion is greater than that of the diffractive-dispevsion, so that longitudinal and lateral chromatic aberrations due to the wavelength bandwidth of the pulse laser are canceled and made sufficiently small. Therefore, a focused spot becomes smaller to thereby improve processing accuracy.
(2) Further, chromatic aberrations and pulse extension pertinent to the laser pulse are corrected by the combination of diffractive and refractive optical elements. According to the method, diffractive-dispersion and refractive-dispevsion are caused by the diffractive and refractive optical elements, respectively. These two kinds of dispersion matelaizes in a counter opposite manner and the magnitude of the diffractive-dispersion is greater than that of the refractive-dispersion, so that chromatic aberrations and pulse expansion due to the wavelength bandwidth of the laser pulse are suppressed to a sufficiently low level. Therefore, a focused spot becomes smaller to thereby improve processing accuracy.
(3) In a laser processing apparatus of the present invention having a pulse laser focusing optical system, for performing the processing by impinging a beam delivered from the focusing optical system on a work, the focusing optical system includes at least a pair of a diffractive surface and a refractive surface. According to the apparatus, the pair of the diffractive surface and the refractive surface functions to cause the effects explained in the items (1) and (2), thereby the processing can be performed with a pinpoint accuracy.
(4) The focusing optical system may be composed of a refraction lens having a diffractive surface.
(5) Further, the focusing optical system may be composed of a sheet having a refractive surface and a refraction lens.
(6) Another method of the present invention includes the steps of branching a laser pulse into a plurality of beams by a branching diffraction element, correcting a chromatic aberrations caused to the plurality of branched beams through a focusing optical system, and focusing and impinging the plurality of branched beams whose chromatic aberrations have been corrected, on a work. According to the method, the distortion in shape of the array of focused spots obtained by focusing the array of pulse beams branched by diffraction element, which is caused by chromatic aberrations, can be prevented.
(7) A focusing diffraction element having a focusing action may be used as the focusing optical system, and the distance between the branching diffraction element and the focusing diffraction element may be set approximately equal to the focusing length of the focusing optical system. According to the arrangement, the lateral aberration caused on the array of beams branched by the branching diffraction element is removed, thereby the shapes and sizes of the array of focused spots formed from the array of pulse beams branched by the branching diffraction element can be made approximately the same. Therefore, a plurality of processed works having approximately the same shape and size can be obtained on a work.
(8) Further, an optical system composed of at least two groups each having refracting and diffracting actions may be used as the focusing optical system, and the position of the principal plane of the optical system may be best adjusted according to the wavelength band of the pulse laser. According to the arrangement, lateral and longitudinal aberrations caused on the array of beams branched by the branching diffraction element are removed, thereby the shapes and sizes of the array of focused spots formed from the array of pulse beams branched by the branching diffraction element can be made approximately the same. Therefore, a plurality of processed marks having approximately the same shape and size can be obtained on a work.
(9) Another apparatus of the present invention includes a laser generator for generating laser pulses a branching diffraction element for branching the laser pulse generated by the laser generator into an array of beams, and a focusing optical system for focusing the array of laser beams branched by the branching diffraction element after correcting chromatic aberrations on the branched beams. The effect explained in the item (6) can be achieved by the apparatus.
(10) A focusing diffraction element having a focusing action may be provided as the focusing optical system, and the distance between the branching diffraction element and the focusing diffraction element may be set approximately equal to the focusing length of the focusing optical system. The effect explained in the item (7) can be achieved by the apparatus.
(11) An optical system composed of at least two groups each having refracting and diffracting actions may be disposed as the focusing optical system. The effect explained in the item (8) can be achieved by the apparatus.
(12) Note that a refraction lens having at least one diffractive surface may be provided as a group of the optical system having the refracting and diffracting actions.
(13) At least one diffractive surface and a refraction lens formed in a member different from a member to which the diffractive surface is formed may be provided as a group of the optical system having the refracting and diffracting operations.
(14) The optical system composed of the two groups may be composed of a first group and a second group, the diffractive surface of the first group may have negative power, the refraction lens of the first group may have positive power, the refraction lens of the second group may have negative power, and the diffractive surface of the second group may have positive power, respectively.
(15) Another method of the present invention may shape the spatial intensity distribution of the pulse laser to a predetermined shape by a diffraction element. According to the method, laser pulses suitable for the processing can be simply obtained by replacing the diffraction element.
(16) Another apparatus of the present invention may include a diffractive pulse shaper for shaping the spatial intensity distribution of the pulse laser to a predetermined shape. The effect explained in the item (15) can be achieved by the apparatus.
(17) Further, a phase function for shaping the spatial intensity distribution of the pulse laser beam to the predetermined shape may be multiplexed to a phase distribution for branching the pulse laser beam. According to the arrangement, laser pulses can be branched and shaped by a single diffraction element, which contributes to the reduction in the size and cost of the apparatus.
(18) Another method of the present invention includes the steps of diffracting a pulse laser beam by a diffraction element, correcting chromatic aberrations caused on the pulse laser beam through a focusing optical system, and moving the focusing position of the pulse laser beam by rotating the diffraction element. According to the method, processing can be performed with a pinpoint accuracy by the diffraction element and the determination of processing position by rotation of the diffraction element, in addition to the correction of chromatic aberrations.
(19) Note that the pulse laser beam may be branched into a plurality of beams, and the plurality of branched beams may be focused and arrayed in a pattern. According to the arrangement, processing speed and a processing yield can be improved.
(20) A focusing diffraction element having a focusing action may be used as the focusing optical system, and the distance between the branching diffraction element and the focusing diffraction element may be set approximately equal to the focusing length of the focusing diffraction element. According to the arrangement, lateral chromatic aberration caused on the plurality of beams branched by the branching diffraction element is removed, thereby the shapes and sizes of the array of focused spots formed from the plurality of pulse beams branched by the branching diffraction element can be made approximately the same.
(21) Further, an optical system composed of at least two groups each having refracting and diffracting actions may be used as the focusing optical system, and the position of the principal plane of the optical system may be best adjusted according to the wavelength band width of the pulse laser. According to the arrangement, lateral and longitudinal chromatic aberrations caused on the plurality of beams branched by the branching diffraction element are removed, thereby the shapes and sizes of the array of focused spots formed from the plurality of pulse beams branched by the branching diffraction element can be made approximately the same.
(22) Another apparatus of the present invention includes a laser generator for generating a pulse laser beam, a diffraction element for diffracting the pulse laser generated by the pulse generator, a focusing optical system for correcting chromatic aberrations aberration due to the wavelength bandwidth of the pulse laser caused on the diffracted pulse beam by the diffraction element and focusing the diffracted pulse laser beam whose chromatic aberrations have been corrected, and a rotation unit for rotating the diffraction element. The effect explained in the item (18) can be achieved by the apparatus.
(23) Note that the rotation unit may rotate and move the focused spot of the laser beam diffracted by the diffraction element.
(24) A branching diffraction element for branching the pulse laser beam generated by the laser generator into a plurality of beams may be provided, and the branching diffraction element may produce a plurality of branched beams. According to the arrangement, a plurality of focused spots are provided, thereby processing speed can be increased.
(25) Note that the optical axis of the laser beam incident on the diffraction element may be set to agree with the center of rotation of the diffraction element.
(26) Further, the optical axis of the laser beam incident on the diffraction element may be off-set from the center of rotation of the diffraction element.
(27) A focusing diffraction element having a focusing action may be provided as the focusing optical system, and the distance between the diffraction element and the focusing diffraction element may be set approximately equal to the focusing length of the focusing diffraction element. The effect explained in the item (20) can be achieved by the apparatus.
(28) An optical system composed of at least two groups each having refracting and diffracting actions may be disposed as the focusing optical system. The effect explained in the item (21) can be achieved by the apparatus.
(29) Note that a refraction lens having at least one diffractive surface may be provided as a group of the optical system having the refracting and diffracting actions.
(30) Further, at least one diffractive surface and a refraction lens formed in a member different from another member to which the diffractive surface is formed may be provided as a group of the optical system having the refracting and diffracting operations.